In order to gain further insight into the pathogenesis and immunological factors involved in Marek's disease (MD), we will continue studies with Marek's disease virus (MDV) induced transplantable lymphomas in inbred strains of chickens. Various parameters concerning the identification of MDV transformed cells during in vivo serial passage will be studied. Antigenic make-up and immune response to MD tumors during successive passage and differences between tumors induced by MDV strains of low and high virulence, with regard to transplantability and cellular composition, will be analyzed. Methods to be employed include in vitro cytotoxicity assays using 51Cr release and cell membrane immunofluorescence with labelled antibody in vitro and in vivo assays to determine a possible role of "arming" antibody in MD, and the use of bursectomized and/or thymectomized birds for functional assays of T and B cells in tumorigenesis. Possible differences in turkey herpesvirus (HVT) vaccination effects, as related to immunogenicity of transformed cells, will be examined to determine whether HVT vaccination induces immunity to transformed cells of differing antigenicities or protects at a level prior to malignant transformation and cell proliferation.